The invention relates to “active implantable medical devices” as defined by Directive 90/385/EEC of 20 Jun. 1990 of the Council of the European Communities, specifically to devices that continuously monitor heart rhythm and deliver to the heart if necessary stimulation, resynchronization and/or defibrillation pulses in cases of arrhythmia detected by the device.
The invention relates especially, but is not limited to, those devices that are in the form of an autonomous capsule intended to be implanted in a heart chamber (atrium or ventricle, right or left). These capsules are free of any mechanical connection to an implantable (such as a housing of the stimulation pulse generator) or non-implantable (external device such as programmer or monitoring device for patient remote monitoring) main device. For this reason they are called “leadless capsules” to distinguish them from electrodes or sensors disposed at the distal end of a conventional probe (lead), which is traversed throughout its length by one or more conductors connecting the electrode or sensor to a generator connected to an opposite, proximal end of the lead.
EP 2394695 A1 (Sorin CRM),now EP 2394695 B1, describes an autonomous intracardiac capsule, and a method to implant it to the selected detection/stimulation site and reposition it if necessary.
Note, however, as will be understood by reading the description, that the autonomous nature of the capsule is not inherently a necessary feature of the invention, and that the latter can be both applied to capsules permanently mounted at the distal end of a lead.
An implantable capsule includes a body housing the main components of the device (electronic circuits, power source, stimulation electrodes, etc.) and a base secured to the body and rigidly supporting methods for fixing to the wall.
In the case of cardiac leads, two types of fasteners are known and conventionally employed: fixation with “barbs” is the oldest and is still used marginally, but the leads based on a fixing screw have supplanted barb leads and currently represent the majority of the market. They allow a generally robust and effective fixation.
The screw is a projecting helical screw extending axially the capsule body and adapted to penetrate the heart tissue by screwing in the implantation site, in the same method as the conventional screw leads.
However, the fixing of such devices is still a critical point to the extent that accidental detachment of the capsule would cause the latter to be released into the heart chamber and then transported by the blood in the venous or arterial system. Complication risk to the patient would be extremely high, as well as the risk of cardiac system injury which can be generated by the end of the fastening system or other projecting regions of the implant such as a needle electrode or a projecting ridge.
More than a lead device, an autonomous device undergoes stresses and movements generated by the heart wall, as it does not benefit from the axial holding force from the lead body.
To fulfill its permanent anchoring function, the fastening system must also include a function of irreversibility. That is to say it can be removed from the heart wall only by the doctor's voluntary action and according to a predefined method, but in no case by repeated movements or vibrations of the heart, or by modification of heart muscle due to the disease or tissue aging.
WO 2012/051235 A1 discloses reverse rotation prevention methods which implement arrangements with protruding spikes formed directly on the screw and oriented in an opposite circumferential direction to the direction of screwing, or protruding spikes penetrating the tissues at the base of the screw. This implies significant damage to all tissues crossed or adjacent during implantation of the implant.